Moistureproof transparent material



Patented Apr. 28,1936

UNITED STATES MOISTUREPROOF TRANSPARENT MATERIAL Edouard M. Kratz, Gary,Ind., and William C. Wilson, Chicago, Ill., assignors, by direct andmesne assignments, of one-half to Pyroxylin Products, Inc., Chicago,111., a corporation of Illinois, and one-half to Marbo ProductsCorporation, Chicago, 111., a corporation of Delaware No Drawing.Application April 1, 1933, Serial No. 664,042

5 Claims.

This invention relates to a moisture proof transparent sheet materialthat is especially adapted for use in wrapping articles. Moreparticularly, the invention relates to a relatively moisture proofmaterial having a protein base and a cellulose ester coating firmlyadhering thereto provided with a moisture proofing agent, suchas ahigher fatty acid.

It has heretofore been proposed to prepare sheet material containingcasein as a principal ingredient and although it has been alleged thattransparent, flexible sheets of casein material can be made, to the bestof our knowledge, no one has heretofore developed commercially asatisfactory sheet having a casein base that possesses moistureresistant properties adapting it for wrapping purposes.

Furthermore, while various types of moisture proof coating compositionshave been known in connection with the coating of transparent sheetmaterial, such as regenerated cellulose and the like, no commerciallysatisfactory sheet material having a transparent casein base and amoisture proof coating thereon has been developed to the best of ourknowledge. In order to be commercially satisfactory for use as awrapping material, the sheet must have a base that is relatively thin,tough, pliable, flexible and transparent.

In the manufacture of sheets having a casein base, it has heretoforebeen impossible, practically, to obtain a base film having these desiredcharacteristics, largely because of the inherent properties of caseinand because of the difiiculty in obtaining a casein material that isclear and homogeneous and free from particles or specks. In a very thinsheet, solid particles of non-colloidal dimensions would produce aroughened surface that would not only decrease the transparency of thefilm but would make it difiicult to provide a continuous, smooth,moisture proof coating over such surface.

Furthermore, in order to make a commercial sheet having a casein base,it is necessary to provide a continuous method of casting long webs ofthe material. Suitable apparatus for and a method of casting transparentweb material having a casein base aredescribed in the copendingapplication of one of us, Serial No. 611,329, filed May 14, 1932.

Owing to the naturally unstable properties of casein with respect toatmospheric conditions, such as temperature and relative humidity,sheets made from casein, unless the inherent properties of the caseinare modified, are unsatisfactory for many wrapping purposes and must beprotected by a moisture resistant coating.

Although it might be possible to utilize some of the known moistureproof coating compositions in the coating of casein base material, suchcom- 5 positions are not best adapted for this purpose. It might bepossible, for instance, to use coating compositions having anitrocellulose base and containing in addition to a plasticizer and agum, relatively small proportions of paraflin, but such 10 coatingcompositions, in general, have objectionable features. Their applicationrequires drying temperatures above the melting point of the parafiin waxemployed to prevent blushing or ,other surface phenomena that impair theap- 15 pearance of the sheet and are deleterious to the base material,or casein.

We have now found that coating compositions containing a cellulose esterand a higher fatty acid may be satisfactorily used in the coating of 20casein base material to render the same relatively moisture proof. Thenecessity of providing a coating having a high resistance to thetransmission of water vapor is apparent in connec-' .tion with the'useof this type of wrapping. ma- 25 terial for cigars, frozen foods, andthe like. It is also necessary that the coated material be relativelythin, transparent and have a high degree of flexibility over a widetemperature and humidity range.

It is therefore anobject of this invention to provide a relatively thin,transparent, moisture proof and flexible sheet especially adapted forwrapping purposes, having a casein base pro vided with a cellulose estercoating firmly bonded 35 thereto and containing a-moisture repellentagent.

It is a further important object of this invention to provide acomposite, integral sheet for wrapping purposes having a hardened caseinbase and a moisture resistant coating firmly bonded thereto. a

It is a further important object of this invention to provide arelatively moisture proof coating for casein base materials wherein ahigher plasticizer, a gum and a higher fatty acid, such as stearic acid.

It is a further important object of this invention to provide arelatively thin, transparent, moisture proof and flexible wrappingmaterial having a hardened casein base and having a coating on one orboth sides thereof firmly anchored thereto and comprisingnitrocellulose, a gum, a plasticizer, a higher fatty acid and atempering oil for maintaining the fatty acid in a state ofcompatibility.

Other and further important objects of this invention will becomeapparent from the following description and appended claims.

The base of the flexible wrapping material of our invention may be asuitable protein, such as casein. Casein is preferred because of itsrelative cheapness, ready source of supply, high transparency, goodkeeping qualities and the high form of purity commercially available. Weprefer to use a lactic acid casein of low ash content that has beendigested with acid and subsequently rendered alkaline with a causticalkali until a clear solution results. However, it will be understoodthat any casein, such as a rennet casein, can be used which is capableof producing a clear solution of a caseinate, free from dirt, foreignparticles and particles of non-colloidal proportions.

According to our method, a suitable aqueous caseinate solution may beprepared to contain from 13 to 14% of alkali metal caseinate by weight.To this solution may be added a suitable softening agent, such asglycerine, a glycol, e. g.

' diethylene glycol, a deliquescent salt, e. g. potassium acetate, orthe like.

In casting the casein material in the form of a continuous web, weprefer to use the method and apparatus disclosed in the copendingapplication of Kratz and Heckel entitled Method and apparatus for makingtransparent sheet material, Serial No. 630,406, filed August 25, 1932.According to that disclosure, the casein solution, comprising 13 to 14%of alkali metal caseinate and from 3 to 4% of glycerine, or othercorresponding amounts of a softening agent, by weight, the balance beingwater, is mixed in the desired proportions with a formaldehyde solution,preferably a 10% solution (by weight) of formaldehyde in water. Asuitable proportion is 1 part of 10% formaldehyde solution with 22 partsof the aforesaid casein solution.

The casein and formaldehyde solutions are separately prepared and mixedjust prior to introduction into the coating machine. For this purpose, apond of the mixed casein and formaldehyde solution may be suitably builtup by means of a weir, doctor or the like on a coating roll to obtain afilm of the desired thickness and this film then transferred to acontinuous, traveling carrier or belt. For best results, a belt having afabric base and a proxylin coating or surface, which may be eitherplain, printed or embossed, is used. After the casein-formaldehydesolution has been cast onto the traveling belt, the belt is passedthrough a heated drierto permit the film to set and is then carriedthrough a festoon drier to thoroughly dry the film.

As an alternative to mixing the formaldehyde and casein solutiontogether prior to casting on a traveling belt, the casein solution aloneor in a suitable solvent or carrier therefor may be first cast on thebelt and then the film exposed to formaldehyde gas to harden the casein.

It is also feasible to prepare an ammonia neutralized casein solutioncontaining the proper proportion for formaldehyde and then, by theaddition of a small quantity of caustic soda, to cause the solution toset quickly when cast. A pH of about 6.7 is desirable for this type ofreaction.

After the film has been dried and preferably conditioned, by controllingthe relative humidity of the air in the festoon drier, the film isstripped from the carrier belt and wound into rolls.

While we have here referred to the base film as being a casein or alkalimetal caseinate base, it will be understood that neither casein alonenor an alkali metal caseinate is of itself a suitable base but that thecaseinate material must be hardened, since casein or caseinate does notset by itself, as does gelatin. The action of formaldehyde in hardeningcasein produces a complex substance, which, depending upon the amount offormaldehyde used, is relatively water insoluble and relatively brittle.

In order to overcome the natural brittleness of the casein-formaldehydecomplex body, a suitable softening agent, such as glycerine, must beemployed. Glycerine, being a hygroscopic substance, tends to retain andmaintain a moisture content in the casein base film such that the filmpossesses a desired degree of flexibility and pliability throughoutcomparatively wide fluctuations in atmospheric temperature and humidity.Other hygroscopic substances, both liquid and solid, examples of thelatter being deliquescent salts, can be used.

The coating equipment may be of standard construction, such as an airfloated coating device, a tower coater or a vertical coating machine. Inthe first mentioned equipment, the liquid coating composition is appliedto the base film by means of a roll doctor or by other suitable meansand the coated material is kept suspended by currents of air for aconsiderable distance until the coating has had an opportunity to dry,after which the coated material is wound up into rolls of the desiredsize.

A typical example of a suitable dry base composition for coating thecasein base material is as follows:

Parts dry weight Regular soluble nitrocellulose sec. viscosity) 52 Estergum 20 Dibutyl phthalate 10 Stearic acid 8 Paraffin oil 4 A suitablesolvent for use with a nitrocellulose base such as this is as follows:

Parts by weight Toluol 133 Solvent naphtha 53 Butyl acetate 40 Ethylacetate 85 Butanol (butyl alcohol) 10 The proportion of solvent mixtureto the dry base may be widely varied to meet the conditions imposed uponthe coating operation, as is well known to those skilled in the art. Ingeneral, the percentage of solvent mixture to dry base, by weight, willvary between 5 and 40% and preferably will be between 15 and 25% underusual coating conditions.

In the moisture proof coating other cellulose esters, ethers orderivatives may be used, such as cellulose acetate, cellulose ether,such as ethyl cellulose, and the like.

The purpose of the ester gum is largely to give body to the coating andto serve as an anchoring or bonding agent to cause the coating to adherefirmy to the casein base film. The ester gum also has a certain degreeof moisture and water resistance properties. It will be understood thatin place of ester gum, any suitable gum orresin, either natural orsynthetic, may be used, such as dammar gum, phenol-formaldehydecondensation products or partial condensation products containing rosin,pontianac, sandarac and other fossilgums and resins.

The dibuiyl phthalate serves as a plasticizer to impart the desiredflexibility and softening effect to the coating film. While we prefer touse dibutyl phthalate, it will be understood that other phthalates maybe used and also other plasticizers, such as ortho tricresyl phosphate,triphenyl phosphate, and the like.

While we have given stearic acid as the preferred higher aliphatic fattyacid, other saturated fatty acids having about the same number of carbonatoms to the molecule or a higher number may be used with more or lesssatisfactory results, such as myristic, lauric, palmitic, and the like.We prefer to use stearic acid both because of its ready availability andbecause of its relative cheapness. Furthermore, when a coatingcomposition containing stearic acid is employed, it is unnecessary touse temperatures during the drying operation in excess of the meltingpoint of stearic acd, but rather instead, temperatures considerablylower than the melting point of stearic acid may be employed, as forinstance from 50 to F.

While it is known that paraffin wax and other solid hydrocarbons, aswell as true waxes may be used in coating compositions of this generalcharacter, the higher fatty acids such as herein specified are not theequivalent of parafiin or of .waxes such as have heretofore been used.In

the use of parafiin and waxes, it has been considered necessary toemploy temperatures during the drying operaton about or above themelting point of the paraflin or wax employed, since otherwise there isa tendency for the film to blush, that is, become slightly milky oropaque, or present a streaked appearance. Also the use of waxes resultsin extremely slow drying of the coated sheet. These phenomena do notoccur where stearic acid, or other suitable higher fatty acids, isemployed, especially where a tempering agent, such as paraffin oil isadditionally used.

While we have found that various mineral oils. such 'as the so-calledrefined mineral oils, Russian white oil, and the like, may be used as atempering agent in connection with the higher fatty acids, we prefer touse a wax-free grade of nonvolatile and odorless parafiin oil. Thisparaffin oil has been produced by cooling to a temperature of m nus 50C. in order to free the oil from any paraflin or other normally solidsubstance.

In general, any petroleum base oil of the gen? eral character indicatedmay be used. Paraffin oil acts to prevent the stearic acid from spewing,so

that larger proportions of stearic acid can be used to advantage in themoisture proofing of the sheet.

If no tempering agent is used, the stearic acid has a tendency tocrystallize in the coating film and this is objectionable as it reducesthe transparency of the film and also the resistance of the film to thepenetration of moisture vapor. While excessive amounts of paraffin oilwould be ob,- jectionable because giving an oily or greasy appearanceand feel 'to the surface of the coated material, small amounts of theparafiin'oil serve to disperse the stearic acid and to prevent it fromspewing out.

The use of water soluble orwater miscible compounds in the solventmixture should be avoided or reduced to a minimum so far as possible,where a clear transparent sheet is desired, since relatively largeproportions ofwater soluble or watertmiscible compounds in the solventsused tend to give a milky or opaque appearance to the coated sheet.

The exact phenomenon by which good anchoring qualities in the coatinglayer are obtained is not fully understood, but it is apparent that morethan mere physical contact and adhesion result ing therefrom isrequired.We have found that by using a coaiing composition of the preferredformula given above, we can obtain a coated material that will notdelaminate when immersed in water over a considerable period of time.While we have attributed the good anchoring qualities of our coatingcomposition to the use of a gum or resin, we also recognize that thequantity of plasticizer has a modifying effect.

After the casein base-film has been coated with a composition such asabove described, and. the coated material has been dried to remove thevolatile solvents, the dried material may be conditioned to give it thedesirable flexibility. Owing to the fact that the coating apparatus mustnecessarily be enclosed in order to permit economical the base film, itis possible to use moderately conditioned low temperature air, as forinstance, air between 50 and 120 F. If parafiin wax were used in placeof the stearic acid, temperatures of from 120 to F, would be necessaryand the sheet would have to be carefully conditioned after drying.

It will be understood that in giving exact pro portions of the variousingredients of the casein base material, of the dry base constituents ofthe coating composition and of the solvent mixture, we do not intend tolimit our invention to these specific proportions, since they may bevaried considerably without impairing the general qualities of the sheetmaterial. For instance,'on the basis of 36 parts by weight ofnitrocellulose, we may use between 10 and 30 parts of-a suitable gum orresin, between 5 and 15 parts of a suitable plasticizer, depending uponthe plasticizer selected and the degree of flexibility desired, between4 and 12 parts of a higher fatty acid, and between 2 and 6 parts of atempering oil. Likewise, depending upon the characteristics of the sheetdesired, the gum or resin may be entirely omitted.

In the case of the solvent, equivalents of the different ingredients maybe used and the proporlate. Under "some circumstances, either the butylacetate or the ethyl acetate may be omitted and a corresponding quantityof the other employed.

Other alcohols than butyl alcohol, such as ethyl or propyl alcohol,maybe used.

One of the important features of our invention is the use of a higherfatty acid in the form of the free acid, rather than in the form of afat, such as a glyceride or other alcohol ester. In general, fats, bywhich is meant a glyceride of one of the higher fatty acids, are notsuitable for our purpose, since it is diflicult to prevent a fat fromspewing out and practically impossible to keep it ina highly dispersedcondition. We prefer to use the free fatty acids for the reason thatthey permit the use of generally lower temperatures during the dryingoperation and have equally as good or better moisture resistantproperties. The attendant dangers of high temperature drying are avoidedby the use of free fatty acids.

By moisture vapor proof sheet, we mean asneetlike material which willnot transmit more than 1.50 grams of water vapor per 100 square inchesin 24 hours at 70 F., when measured by the method as described by AllenAbrams, page 24 of The Paper Mill, volume 8, No. 38 (September 20,1930). The coated sheets of this invention will, in general, transmitbetween 0.40 and 0.75 gram of water vapor per 100 square inches in 24hours at 70 F. when tested by this method.

While, as is obvious, for a given film composition the degree ofresistance to the transmission of water vapor is roughly proportional tothe thickness of the coating, the limits of thickness for coatings ontransparent sheet material for wrapping purposes are relatively welldefined and, in general, for commercial purposes, the coating should notexceed about 20% of the thickness of the base material. If this limit isexceeded, not only is the cost-of the coated material raised to aprohibitive point, but the coated material is rendered less pliable andless flexible to an extent that makes its use for wrapping purposes lesssatisfactory.

Although for the great majority of uses, the coated material of ourinvention is preferably colorless and transparent, it will be understoodthat suitable coloring agents, dyes and the like, may be added to thecasein base fllm if a colored product is desired.

We are aware that numerous details of the process may be varied througha wide range without departing from the principles of this invention,and we, therefore, do not purpose limiting the patent granted hereonotherwise than necessitated by the prior art.

We claim as our invention:

1. A thin, transparent and relatively moisture proof flexible sheetadapted for use in wrapping articles comprising a casein base having acellulose ester coating thereon containing stearic acid.

2. A thin, transparent and relatively moisture proof flexible sheetcomprising a casein base having a plasticized cellulose ester coatingthereon containing stearic acid and a tempering oil.

3. A thin, transparent and relatively moisture proof flexible sheetadapted for use in wrapping articles comprising a casein base having anitrocellulose coating thereon containing stearic acid, a gum, atempering agent and a plasticizer.

4. A relatively thin, transparent moisture proof and flexible sheetadapted for wrapping purposes comprising a hardened casein basecontaining a softening agent and a coating on said base firmly adheringthereto, said coating containing nitrocellulose, ester gum, dibutylphthalate, a mineral oil and stearic acid.

5. A thin, transparent and relatively moisture proof flexible sheetadapted for use in wrapping articles comprising a casein base having acoating thereon consisting of 36 parts by weight of nitrocellulose,between 10 and 30 parts by weight of a gum, between 5 and parts byweight of a plasticizer, between 4 and 12 parts by weight of a higherfatty acid and between 2 and 6 parts by weight of a mineral oil.

EDOUARD M. mRA'I'Z. WILLIAM C. WILSON.

